High molecular weight polysaccharides (PS) have been isolated from saline extracts and cultural supernatants of Psuedomonas aeruginosa, Fisher immunotypes 1 and 2. The antigens are composed of carbohydrate and water, and are purified by a series of chemical and physical treatments, which include removal of nucleic acids by precipitation with cetavalon, removal of lipopolysaccharide (LPS) by hydrolysis with acetic acid, removal of lipids by chloroform extraction, removal of protein by phenol extraction, and chromatography on Sephadex G-100 molecular seive gels. Polysaccharides have a molecular weight of between 100,000 and 350,000 daltons, and are non-toxic and non-pyrogenic in animal tests. The antigens are also expressed on the LPS molecule, but differ from the polysaccharide present on LPD in that the polysaccharide antigens are larger and contain the sugars galactose and arabinose, which LPS does not contain. Immunotype 1 polysaccharide, immunizes mice against challenge with live, homologous organisms, as well as against challenge with heterologous immunotype organisms. Polysaccharides from other Pseudomonas aeruginosa strains will be prepared, and evaluated for size, chemical composition, toxicity, pyrogenicity, and immunogenicity. From these data a vaccine will be formulated that will contain a representative number of polysaccharides antigens from Pseudomonas aeruginosa strains, and tested in mice for efficacy against a representative cross section of clinical isolates of Pseudomonas aeruginosa. Confirmation of active immunization will be done employing passive transfer of rabbit serum to the polysaccharides antigens. The nature of defenses to Pseudomonas aeruginosa will be evaluated in terms of immunity to polysaccharides and lipopolysaccharide. This will be accomplished by evaluation of serum antibody levels and function in different serological assays. A model for susceptibility to Pseudomonas aeruginosa will be developed.